The Population Studies Center (PSC) of the University of Pennsylvania (Penn) was established in 1962 and has enjoyed core support from NICHD under the P30 and R24 mechanisms continuously since 1978. The PSC at Penn comprises 47 program scientists working in the last year on $39.7 million in extramurally funded research projects, with emphasis on: the health and well-being of populations;human resources and endowments;innovation in methods;international population research;policy evaluation;growth and structure of populations;and networks in populations. Their research is highly visible: Penn program scientists'rank 1st among peer population studies centers in publications in leading journals in economics, demography, sociology, and medicine combined during the past five years, and 2nd in citations to these articles. They rank 3rd in number of NIH grants. The PSC is a major national resource for our understanding of the dynamics of human populations, from fertility and mortality to the effects of public programs on health and inequality. Penn program scientists are heavily represented on national scientific boards and commissions, and have earned numerous honors reserved for recognized leaders in understanding the nation's health (e.g., 5 in the Institute of Medicine). Their work has had demonstrable impacts on public programs and legislation in the US and abroad. The PSC is centered on a core of economists, demographers, and sociologists, but has successfully integrated others with interests in population studies and population health, from biological anthropology to medicine to statistics to history. The shared intellectual interests and complementary skills create economies of scale in research that would be lost in the absence of the PSC. We request renewal of essential support for shared services, primarily in research cores dedicated to (a) administrative services for organizing large-scale externally-funded research activities and (b) the computing and information infrastructure for cutting-edge, collaborative scientific activity;and also for (c) developmental activities to generate new projects, recruit new population scientists, and upgrade the skills of others, and (d) a one-time effort to generate a set of best practices relating to human subjects issues arising in international and interdisciplinary research. The PSC, in addition to the monies received from NIH, is generously supported by Penn, especially in the provision of full salary support for population scientists.